1. Field of the Invention
The present invention relates to a generator control unit which controls a generator.
2. Description of the Related Art
A power generation unit which combines a field winding-type three-phase synchronous generator and a diode rectifier, referred to as an alternator, has conventionally been used as an on-board generator. The alternator is a power generation unit which controls field voltage to control generator output to a desired value. Since the alternator makes it unnecessary to control a generator armature current by means of an inverter or a chopper, it provides a low-manufacturing-cost advantage and has been widely used as a generator for an automobile.
To control an output voltage of the alternator, it is common to perform feedback control on the field voltage through comparison of a detected DC output voltage with a DC voltage command value by use of a PT controller or the like (refer to, for example, JP-A-60-106338).
However, the method of controlling the field voltage uses a constant feedback gain of the PI controller and therefore is inferior to an armature control type in DC voltage control performance; its DC voltage response performance largely depends on rotational speed and load fluctuations.
As a method of performing PI control according to a deviation between a DC voltage value and a DC voltage command value, a technique of changing the feedback gain based on operating conditions of a power generation unit is known (refer to, for example, JP-A-60-106338). For example, effects of rotational speed fluctuations of the three-phase synchronous generator on the DC current is large in a low rotational speed region and small in a high rotational speed region. Therefore, the amount of feedback gain to be given is small in a low rotational speed region and large in a high rotational speed region. This makes is possible to restrain fluctuations of generated electric power caused by rotational speed fluctuations. Further, since an increase in the field current of the three-phase synchronous generator saturates the change of the DC current, the gain is increased according to an increase in the field current so that the power generation unit can supply electric power required by an electric load in a highly responsive manner.